None.
The present invention relates to ionization detector devices. More specifically, the present invention relates to an apparatus and a method of making a hermetically sealed discharge ionization detector having a small detection volume.
As illustrated in FIG. 1, an ionization detector 100 typically comprises a body 102 having a first chamber 110 for generation of ionizing particles and a second chamber 120 connected to the first chamber 110 for receiving a sample gas 122. The sample gas 122 is conveyed in a carrier gas and is provided to the second chamber 120 by a conduit 130 which typically is provided in the form of a separation column. The first chamber 110 includes a source of ionizing particles (not shown), such as a radioactive source or an electrical discharge, and is typically swept by a detector or carrier gas 112 selected from the class of known noble gases. The presence of the detector gas 112 in the first chamber 110 causes ionizing particles, in the form of photons and metastables, to be produced. The flow of the detector gas 112 from the first chamber 110 to the second chamber 120 causes the ionized particles to be mixed with the sample gas 122, thus causing the sample molecules of interest, considered herein as analytes, to be ionized. The second chamber 120 includes electrodes 124, 126, 128 for detecting the ionized sample molecules by use of an electrometer circuit (not shown) connected to the electrodes 124, 126, 128.
Detector sensitivity may be measured in a plot of detector response versus analyte concentration or analyte quantity. The range over which the detector sensitivity is constant is called the linear dynamic range, and the entire range over which the response is variable with analyte concentration or quantity is called the dynamic range of the detector. The upper limit of the dynamic range is determined when detector sensitivity falls to an unusable value, typically zero, and the detector is said to be saturated. The lower limit of the dynamic range occurs at a minimum detectable level (MDL).
A discharge ionization detector is an ultra sensitive detector used in gas chromatography. A discharge ionization detector operates by applying a high voltage across discharge electrodes that are located in a gas-filled source chamber. In the presence of a detector gas such as helium, a characteristic discharge emission of photons occurs. The photons irradiate an ionization chamber receiving a sample gas that contains an analyte of interest. Ions are produced in the ionization chamber as a result of photon interaction with ionizable molecules in the sample gas. Helium metastables are also generated in the source chamber and are found to play a role in ionization of the analyte of interest.
A discharge ionization detector can run in a universal, selective, or electron capture mode.
An electrical discharge arc excites the detector gas to glow and give off high energy photons, and excite the detector gas atoms to a metastable level. If the energy of an incoming photon is high enough, photo-excitation can occur to such an extent that an electron is completely removed from its molecular orbital. This is called photo-ionization. A typical photo-ionization reaction resembles the following:
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Also, the metastable atoms can transfer their energy to other molecules during tertiary collisions. If the ionization potential of the analyte molecules is lower than the energy of the photon or the metastable atom, the bombarded or colliding molecule is ionized.
If helium is used as the detector gas, the detector will detect all gases other than helium, because helium has a higher ionization potential than all other gases. This is called the universal mode of detection. UV photon emissions and excited helium atoms in the electrical discharge acquire such energies sufficient to ionize all other gases. When a sample analyte elutes from the chromatographic column into the detector, it becomes partially ionized. The ionized analyte molecules are collected and measured. The electrical current measurement is representative of the analyte presence in the detector. The detector body is usually heated to prevent high boiling sample analytes from depositing on the detector internal surfaces.
Other noble gases with lower ionization potential can be used instead of helium. In this case only sample analytes having lower ionization potential than the noble gas used can be ionized and detected. Thus the detector becomes selective according to analyte ionization potential. This is the selective mode of operation. This can be extremely useful for differentiating between compounds that have similar boiling points but different ionization potentials.
When methane is added to the sample flow, it is ionized by the helium discharge, producing thermal electrons. Any electron-capturing analyte present can capture these thermal electrons and the detector, with some adaptation, can function as an Electron Capture Detector. This is the electron capture mode of operation.
Electron capture detectors for gas chromatography are well known in the art. This type of detector offers high sensitivity and high selectivity towards electrophilic compounds and is widely used for detecting trace amounts of pesticides in biological systems and in food products. Such compounds typically contain halogens which combine with free electrons that are created in the ionization chamber in the detector. The resulting decrease in free electrons in the ionization cell is monitored as an indication of the concentration of the compounds in a sample.
Certain improvements and modifications have been made to ionization detectors in order to overcome certain problems inherent in the prior art. For instance, a funnel-shaped detector cavity, designed to inhibit sample analytes diffusing back towards the ionization chamber as disclosed in U.S. Pat. No. 6,037,179 to Abdel-Rahman, and an ionization detector designed to have an extended detection zone, as disclosed in U.S. Pat. No. 6,107,805 to Abdel-Rahman, both of which are incorporated herein by reference in their entirety, have alleviated or solved certain problems in the prior art, such as analyte diffusion and small linear dynamic range.
Discharge ionization detectors typically have a detector volume of approximately 150 xcexcL. This requires large amounts of analytes, and high gas flow rates. This can be problematic, especially for ultrafast and portable gas chromatographs. Also, because of the ultra sensitivity of a discharge ionization detector, any ambient air (or other detectable gases) leaking into the detector is detected, causing the detector baseline signal to wander. This significantly increases the detector""s noise and worsens the detector""s MDL.
Discharge ionization detectors well known in the art, such as those made by VICI(copyright) and Gow Mac(copyright) Instrument Company, are constructed of several parts mechanically assembled together to form the final detector assembly. This type of construction uses mechanical compression of surfaces to form various seals. The seals can eventually exhibit ambient air leaks when various compressed parts expand and contract as the detector is heated and cooled. Air leaks can also develop as the various compressed parts relax over time and plastically deform. This type of construction is also difficult to miniaturize because high-precision alignment of assembled mechanical parts is extremely difficult to achieve.
What is needed is a discharge ionization detector that has a smaller detector volume and requires no or little assembly. What is also needed is a discharge ionization detector that reduces or eliminates ambient air leaks which can cause increased detector noise and loss of MDL.
The present invention includes a discharge ionization detector consistent with any of its modes of operation including the universal, the selective and the electron-capture modes, as well as a method of making a discharge ionization detector.
The present invention overcomes the problems of high detector volume and ambient air by using a high-precision machined body that constitutes the heart of the detector. In one embodiment, the body is ceramic. Ceramic material of high purity is advantageous in order to minimize the electrical leakage current between the signal electrode and the other electrodes and metallic connectors. Because of high-precision machining, the detector cavity can be made very small. Detectors having a detection volume of about 10 xcexcL or less can be achieved for certain embodiments. In one embodiment, a ceramic body is metallized then brazed to various electrodes and pneumatic connectors thus producing a one-piece hermetically sealed design.
A smaller detector volume allows sample analytes to be quickly swept out of the detector thus enabling the detection of narrower chromatographic peaks. A smaller detector volume enables faster chromatography, and also allows for lower gas flow rate. Lower gas consumption is highly advantageous for portable gas chromatographs.
While multiple embodiments are disclosed, still other embodiments of the present invention will become apparent to those skilled in the art from the following detailed description, wherein is shown and described only the embodiments of the invention, by way of illustration, of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of modifications in various obvious aspects, all without departing from the scope of the present invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not restrictive.